The invention relates to a needle shut-off nozzle for plastic injection moulding compounds with an internal chamber with a spring acting on a shut-off needle in a basic body.
Injection moulding machines for thermoplastic materials, in particular for plastics, principally consist of two components, these being the clamping unit, with which the injection mould is closed and opened, and the injection unit, comprising a heatable injection cylinder, in which the injection moulding compound (plastic) is plasticated and injected into the injection mould via a suitable injection nozzle by means of a pressurisable plunger which moves in the cylinder.
The viscosity of the liquefied plastics varies. Viscous plastics with low melting points can be injected into the mould via simple, so-called open-channel nozzles, i.e. via nozzles through which the injection moulding compound can be injected into the injection mould without resistance. In the case of injection moulding compounds whose melting temperature is very high, as with polyamides, for example, the melt is of low viscosity and escapes as soon as the injection cylinder is raised from the injection mould during the injection cycle.
In order to be able to process plastic melts of low viscosity, shut-off nozzles were designed which only permit passage of the plastic melt during the injection procedure, which takes place under high injection pressure, while escape of the melt from the nozzle is prevented when the injection cylinder is lifted off the mould. The outlet, normally a hole drilled in the tip of the nozzle, is sealed on the inside of the nozzle by a shut-off needle which, in turn, is under spring pressure in a cylindrical, pressure-tight guide. In a different design, the pressure on the shut-off needle is exerted hydraulically or pneumatically via levers located outside the nozzle.
The nozzles designed to date have the disadvantage that the guide of the shut-off needle becomes worn after extended usage. Consequently, plastic melt emerges through the guide of the needle plunger, so-called leakage, which can lead to undesired contamination of the injection moulding machine, material loss and, in extreme cases, pressure loss. In the case of shut-off needles with an internal pressure spring, in particular when disk springs are used, the spring chamber gradually fills with plastic, which forms a crust and can, in particular when disk springs are used, eliminate the effect of the spring. The result of this is an increasingly frequent need to dismantle the shut-off nozzle and clean the spring chamber, this entailing a corresponding machine stoppage. For this reason, large leakage holes have been provided on needle shut-off nozzles with internal spring chamber for some time.